It is common within the electronics industry to use printing methods, such as silk-screening, for the formation of the copper interconnection lines during the manufacturing of printed circuit boards. Although these printing methods are generally satisfactory for most uses, they are characterized by resolution limits that will not meet future requirements for these types of circuits. In particular, these printing techniques are limited to line widths of about 0.005 inches (five mils). It is foreseeable that eventually these printed circuit board interconnects will necessitate line widths and spacings which are significantly smaller, about three mils, than present standards. In addition, it is believed that ultimately the industry will seek line widths and spacings on the order of about one mil.
Electroless copper deposition is a useful method for metallizing electrically non-conductive substrates, such as the alumina substrates commonly employed within a printed circuit board. A catalyzed surface must be present on the electrically non-conductive substrate in order to initiate the electroless deposition. The most often used catalysts consist of palladium with small amounts of tin. Silver can also be substituted for the palladium, however it is generally less preferred since relatively large amounts of the silver must be present to initiate the catalysis of the electroless copper bath. In addition, copper colloids have been used for these types of catalysts.
An advantageous feature of electroless deposition is that after the electrically non-conductive substrate has been catalyzed, there is no need for further use of the special catalyst. The freshly deposited metal surfaces become the electroless catalysts, therefore, the new metal forming on top of the catalyst actually becomes the catalyst for further build-up of metal. Thus the electroless deposition process, once begun, is self sustaining.
However, although the electroless deposition method is advantageous in some respects, there is a significant drawback to its widespread use--that drawback is the requirement that a catalytically active surface be present so as to initiate the electroless deposition. During the electroless deposition, the metal will deposit only where the catalytically active material is present. Therefore, the resolution of the subsequently deposited metal geometries is determined by (and corresponds directly to) the technique employed to deposit the catalyst material.
Therefore what is needed is a method for depositing the metal catalyst prior to the electroless deposition of copper which avoids the shortcomings of the prior art. In particular, the method for depositing the metal catalyst must be capable of providing fine line, high resolution, geometries characterized by line widths on the order of about one mil.